Skip mode coding is one of the important tools in advanced video coding standards. The accuracy of skip mode coding impacts the rate distortion performance and thus the video quality of an encoder. Most video encoders determine the best encoding mode for a particular macroblock. This requires evaluating the cost of encoding the macroblock in the SKIP mode by computing a true prediction error of a SKIPPED motion vector and comparing this cost with the cost of other intra/inter modes. The video encoder then chooses the minimal cost mode. Computation of this SKIP cost involves generating a predicted macroblock using the actual SKIP motion vector as the True motion vector (MV) including a set of standard defined sub-pixel interpolations generating the predicted macroblock based on a fractional pel accurate SKIP motion vector. Recent advances in fast integer pel search using predictor based fast search patterns the fractional pel search have become as computationally expensive as integer pel search. Thus additional fractional pel prediction computations add significant complexity to the encoding.
Another commonly used method employs a nearest integer approximation of SKIP motion vector. This avoids pixel interpolation and evaluates the prediction error at the full pel SKIP motion vector position for use in mode decision. This results in a suboptimal SKIP mode decision and affects the rate distortion performance and video quality of the video encoder.
One other problem faced in video compression is the amount of data transferred between the core CPU and the memory system storing video data being which includes compressed and reference video frames. Common systems do the mode decisions by operating on subsampled frames which are formed by a subset of pixels that are picked up from the normal frame to reduce the amount of these transfers. Prediction errors are computed based on only the pixels available in the subsampled space. Using these prediction errors in mode decisions results in more suboptimality in the model decision.